Such a circuit arrangement is known from DE 10 2013 201 439 A1 and is depicted by way of example in FIG. 1a. In this connection, DE 10 2012 215 933 A1 additionally discloses an electronic ballast for operating at least one first unit, which includes a first cascade of LEDs and a first storage capacitor connected in parallel therewith, and at least one second unit, which includes at least a second cascade of LEDs and a second storage capacitor connected in parallel therewith. The at least one second unit moreover includes a diode that is coupled in series with the parallel circuit including the second cascade of LEDs and the second storage capacitor. The at least one second unit has an actuating element connected in parallel with it that can be operated in the on state, in the off state or linearly. A corresponding actuating element is also arranged in series with the series connection of the first and at least one second unit. A current controller can be used to actuate the actuating elements, the setpoint value being provided in proportion to the time profile of the AC supply voltage.
WO 2012/034102 A1 discloses a system for directly actuating light emitting diodes (LEDs). A chain of LEDs is coupled via an input voltage and has a plurality of separate groups of LEDs. Each of a plurality of switches is coupled in parallel with an associated group of the groups of LEDs for controlling the flow of current through the associated group of LEDs on the basis of a control signal from a control circuit. A switch protection circuit is connected to at least one of the switches. The switch protection circuit is designed to put the switch into an on state in the event of failure of an LED in the associated group of LEDs.
Moreover, the further sphere of DE 10 2012 006 315 A1 proposes an LED lighting device adapted for an AC power supply, having multiple LEDs that form an overall chain, the LEDs being distributed in LED subgroups. There is a supply voltage having alternating amplitude in the LED lighting apparatus, wherein at least two LED subgroups are interconnected in parallel with one another in a lower circuit state of the overall chain and wherein the overall chain has a first on-state voltage in the lower circuit state. A circuit device is designed to interconnect the at least two LED subgroups in series with one another in a higher circuit state, the overall chain having a second on-state voltage, which is higher than the first on-state voltage, in the higher circuit state. In comparison with a circuit arrangement of the type in question, however, increased complexity in terms of circuitry can be noted for a circuit device for selective series/parallel interconnection. Moreover, LED subgroups that are to be connected in parallel must have a matching forward voltage.
In a broader sphere, US 2010/0060175 A1 discloses an apparatus, a method and a system for supplying power to a load, such as a multiplicity of light emitting diodes, for example. An exemplary apparatus has a primary component, a first secondary component couplable to a first load and a second secondary component couplable to a second load. The primary component includes a transformer having a transformer primary winding. The first secondary component includes a first transformer secondary winding, which is magnetically coupled to the transformer primary winding, and the second secondary component includes a second transformer secondary winding, which is magnetically coupled to the transformer primary winding, the second secondary component being couplable in series with the first secondary component by the first or the second load. When it is supplied with power by a current source, the first secondary component has a first polarity of the voltage and is couplable in series with the first load, which is designed to have an opposite, second polarity of the voltage, the voltages essentially cancelling one another out in order to provide a comparatively low resultant voltage level. Unlike in the case of a circuit arrangement of the type in question, the light emitting diodes are not coupled directly, that is to say not DC coupled, in series between a rectified AC supply voltage, however, but rather are arranged in sections, in each case in alternation with a secondary component that is DC isolated from the primary component. This results in greater complexity for the distributed supply of power by the secondary components and the DC isolation by means of a transformer.
In the case of the circuit arrangement depicted schematically in FIG. 1a, a mains AC voltage 701 is connected to two nodes 703 and 704 via a rectifier 702. The node 703 is connected to a node 759 via the series connection of two resistors R1 and R2. The node 759 is coupled to the node 704 via the series connection of three diodes D5, D6, D7 and a nonreactive resistor R3, the cathodes of the diodes D5, D6, D7 pointing in the direction of the node 704. The nonreactive resistors R1, R2, the diodes D5, D6 and D7 and the nonreactive resistor R3 form a voltage divider whose tap is the node 759. The circuit arrangement additionally includes a voltage equalization series impedance 12, which in the present case is embodied as a linear controller that includes two NPN transistors Q1, Q2 in a Darlington arrangement and a nonreactive resistor R5 that is coupled in series with said Darlington stage Q1, Q2. The base of the transistor Q2 is the control connection of the linear controller 12 and is coupled to the node 759. The ratio R3:R5 sets the ratio by which the current through the Darlington stage is larger than the smaller current through the voltage divider including R1, R2, D5, D6, D7 and R3. Hence, the Darlington current is proportional to the voltage between the nodes 703 and 704, as a result of which the current flowing through the whole circuit turns out to be essentially proportional to the mains AC voltage 701. The voltage equalization series impedance 12 could also be embodied as what is known as a constant current source or, in the simplest case, even as an inductance or as a nonreactive resistor, in which case it is possible to dispense with the voltage divider including R1, R2, D5, D6, D7 and R3.
Coupled between the nodes 703 and 704 is a series connection of, in the present case, three LED units LE1, LE2, LE3 and the voltage equalization series impedance 12. The design of an LED unit is depicted below using the example of the LED unit LE3, the design of the LED units LE1, LE2 being essentially identical, differing only in the number of respective LEDs and the resultant dimensioning of the components.
The LED unit LE3 includes the LEDs D300 to D313, accordingly fourteen LEDs, which are connected in series with one another and form an LED cascade. Coupled in series with the LED cascade is a diode D33, the coupling point between the diode D33 and the LED cascade being a first node N31, to which the cathode of the diode D33 is connected. That connection of the LED cascade that is not coupled to the diode D33 is a second node N32, which can subsequently also be referred to as the “output of the LED unit” or “output node of the LED unit”. That connection of the diode D33 that is not coupled to the LED cascade, that is to say the anode of said diode, is a third node N33, which can subsequently also be referred to as the “input of the LED unit” or “input node of the LED unit”. Coupled in parallel with the LED cascade, there may be an optional capacitor C33, which can also be referred to as a buffer capacitor. Coupled between the node N33 and a fourth node N34 is the series connection of a capacitor C32 and a diode D32, the coupling point between the capacitor C32 and the diode D32 being a fifth node N35, to which the anode of the diode D32 is connected. In the present case, the second and fourth nodes N32 and N34 are identical.
The LED unit LE3 additionally includes two electronic switches Q31 and B31, wherein the control electrode of the switch Q31 is coupled to a node N6 via the series connection of a diode D31 and a nonreactive resistor R31. The reference-ground electrode of this switch Q31 is coupled to the node N35, while its main electrode is coupled to the control electrode of the switch B31 via a nonreactive resistor R32. The reference-ground electrode of the switch B31 is coupled to the node N33, while its main electrode is coupled to the node N32: the output side of the switch B31 between its main and reference-ground electrodes corresponds exactly to the path between the input and output nodes of the associated LED unit. Coupled to the output of a higher than the lowest LED unit there is always the input of the LED unit situated directly beneath, to the output of the lowest LED unit the voltage equalization series impedance, and finally to the input of the highest LED unit the node 703.
In the present embodiment, the switch B31 is implemented as a Darlington stage and includes the transistors Q32 and Q33 and also the nonreactive resistors R33 and R34. Instead of the Darlington stage, however, a single transistor may, in particular, also be provided.
The LED units LE2, LE1 are of comparable design, but each include a different number of LEDs. It goes without saying that further LED units may be provided furthermore. In the present case, the LED unit LE2 includes the LEDs D200 to D227, i.e. 28 LEDs, and the LED unit LE1 includes the LEDs D100 to D155, that is to say 56 LEDs. As can clearly be seen, the number of LEDs doubles from LED unit to LED unit starting from the lowest LED unit LE3 up to the highest LED unit LE1.
The second node of the lowest LED unit LE3, in the present case the node N32 (or N34), is coupled to the main electrode of the voltage equalization series impedance 12 acting as a linear controller, while the third node N13 of the highest LED unit LE1 is coupled to the node 703. Coupled between the node N6 and the node 704 is a DC voltage source 14, which is discussed in more detail further below.
By way of example, the circuit arrangement depicted in FIG. 1a has the following components and dimensions: R1=75 kΩ, R2=500Ω, R3=2.5 kΩ, R5=10Ω, R11=200 kΩ, R21=100 kΩ, R31=50 kΩ, R12=1 mΩ, R22=500 kΩ, R32=250 kΩ, R13=R23=R33=10 kΩ, R14=R24=R34=1 kΩ, C12=470 nF, C22=1 ρF, C32=1 ρF, C13=22 ρF, C23=47 ρF, C33=100 ρF, R4=5 kΩ, C2=22 ρF.
The capacitors C13, C23, C33 are of comparatively large design and serve as a buffer capacitor for the LEDs of the respective LED cascade. In this case, it is advantageous that these capacitors need to be designed only for the voltage dropped across the relevant LED cascade and hence not for the full level of the mains AC voltage 701. Accordingly, these capacitors may be embodied in a smaller form and hence in a more space saving manner.
The diodes D11, D21, D31 are optional and can be eliminated if the transistors Q11, Q21, Q31 are designed to have accordingly high electric strength.
Within the voltage divider, the diode D7 is optional, and the diodes D5 and D6 are used to compensate for the base/emitter voltage of the transistors Q1 and Q2. The voltage dropped across the nonreactive resistor R3 therefore substantially corresponds to the voltage dropped across the nonreactive resistor R5. The current through the resistor R5 is accordingly half-sinusoidal. It follows from this that the current through the circuit arrangement follows the input voltage, as a result of which a good power factor is obtained and low EMC interference.
The effect that can be achieved by the dimensioning of the circuit arrangement shown in FIG. 1a is that the switch B11 is operated at a switching frequency of approximately 100 Hz in one example. Flickering that is sometimes perceptible on account of this switching frequency is prevented by the associated buffer capacitor C13. The switch B21 operates at a switching frequency of approximately 200 Hz, for example, and the switch B31 operates at a switching frequency of approximately 400 Hz, for example.
The combination of the capacitor C12 and the diode D12 is a peak value detector for the LED unit LE1 including the LED cascade with the LEDs D100 to D155. Accordingly, the capacitor C22 and the diode D22 are a peak value detector for the LED unit LE2, and the capacitor C32 and the diode D32 are a peak value detector for the LED unit LE3.
The transistors Q11, Q21 and Q31 act as comparators. The manner of operation is described below using the lowest LED unit LE3 by way of example.
The resistor R32 is designed in combination with the capacitor C32 such that the capacitor C32 is discharged only little even during the longest switched-on phase that can be expected for the switch B31. The voltage source 14 prescribes a voltage offset as minimum voltage, for example at a level of 6V, which is meant to guarantee an adequate operating voltage for the voltage equalization series impedance 12. The transistor Q31 compares the voltage 6V with the voltage on the node N35. If the switch B31 is on, then the LEDs D300 to D313 and their series diode D33 are bypassed, but can still continue to be supplied with power from the buffer capacitor C33 in this phase. Nevertheless, the LED unit LE3 is shorted to the outside. Therefore, the node N35 has reduced the potential from N33 by the voltage across the capacitor C32, which corresponds to the forward voltage of the LEDs D300 to D313. In this phase, the diode D32 is not on. Only when the potential on the node N33, that is to say at the input of LE3, and hence also the voltage across the voltage equalization series impedance 12 becomes higher than the forward voltage of all LEDs that LE3 includes augmented by the voltage offset of the voltage source does the comparator Q31 become high impedance, for which reason the electronic switch B31 coupled thereto also switches off immediately. Buffered by the capacitor C32 and supported by the hard potential on the node N33, the potential on the node N35 remains unaffected by this switching-off process, and only D32 turns on and recharges the capacitor C32. The voltage between the nodes N33 and N32, however, changes abruptly to the forward voltage of the LEDs of the unit under consideration, for which reason the voltage across the voltage equalization series impedance 12 is likewise accordingly reduced abruptly. The point of the voltage offset is thus to ensure, even directly after such a switching-off process, that the voltage equalization series impedance 12 does not uncontrollably become high impedance and briefly switch off the whole system, which would also run counter to the logic just described. Each switching process of one of the second electronic switches B11, B21 or B31 shifts not only the operating point of the voltage equalization series impedance 12 but also the operating points of the remaining actuating units for the LED units currently not under consideration.
Concerning the manner of operation: the switching-on time is subsequently assumed to be the beginning of a half-cycle of the AC voltage source 701. Additionally, it is assumed that all switches of the LED units, i.e. the switches Q11, B11, Q21, B21, Q31, B31, are on and all capacitors are charged (steady state). The forward voltage of an LED is assumed to be 3V, and that of a diode and of a transistor base/emitter junction is assumed to be 0.7V in each case.
As a result of the switches that are on, the instantaneous output voltage from node 703 of the rectifier 702 is also applied to the node N32. The nodes N32 and N33 are at the same potential, since the switches Q31 and B31 have been assumed to be on. The voltage provided on the node N6 by the DC voltage source 14 is assumed to be 6V in the exemplary embodiment.
The capacitor C32 is affirmed to be charged to +42V at the beginning of the half-cycle from the previous cycle. These 42V are obtained from the forward voltages of the fourteen diodes D300 to D313, each forward voltage, as mentioned above, being assumed to be 3V. Hence, a potential of −42V is obtained at the node N35.
The potential of the node N6 is raised to 6V by the DC voltage source 14. This results in a flow of current through the diode D31, the resistor R31 and the transistor Q31. The transistor Q31 is on, since a current flows through its base, limited by R31 and driven by the voltage difference of approximately 48V arising from a potential of approximately 6V on the base and a potential of approximately −42V on its emitter. As a result of the transistor Q31 being on, the switch B31 is also on. The current accordingly flows past the LED cascade of the LED unit LE3, i.e. the LED cascade is shorted to the outside and currently not supplied with power. It can nevertheless be energized from the buffer capacitor C33. As agreed, the switches B21 and B11 are also on, which means that the LED cascades of the LED units LE1 and LE2 are likewise currently not supplied with power, that is to say are shorted to the outside. This situation is the starting point for a half-cycle of the rectified mains AC voltage 701.
In the further course of the half-cycle, the potential of the half-cycle rises. On account of the therefore increasing potential on the node 759, the voltage equalization series impedance 12 operating as a linear controller gradually begins to become conductive.
While the switches Q31 and B31 are on, the potential on the node N33 is equal to the potential on the node N32. In the further course of the half-cycle, the potential on the node N33 rises until the potential on the node N35 is approximately 4.6V (potential on the node N6 minus the forward voltage of the diode D31 minus UBE of Q31). As a result of 42V being stored in the capacitor C32, this is accordingly the case when the potential on the node N32 is 46.6V. At this time, the switches Q31 and B31 change to the off state, i.e. the potentials on the nodes N33 and N32 are decoupled. The potential on the node N33 remains at 46.6V, which still also corresponds to the potential of the node 703. The potential of the node N32 begins to fall on account of the conductivity of the voltage equalization series impedance 12 and the switching-off of the switch B31.
Since the voltage equalization series impedance 12, on the basis of appropriate actuation by the voltage divider, wants to maintain the flow of current through the nonreactive resistor R5 in line with what is prescribed by the voltage divider, it is made increasingly conductive, as a result of which the potential on the node N32 falls until the setpoint current has appeared. This is the case when the voltage on the node N32 has fallen to 3.9V. This value follows from the potential on the node N33, which, see above, is 46.6V after the switches Q31 and B31 have switched off, minus fourteen times the diode forward voltage of 3V, minus 0.7V to the forward voltage of the diode D33. This meets the requirement of the current flowing via the LED cascade of the LED unit LE3, for which reason this cascade lights up from this time onward (provided that the optional capacitor C33 is absent; if it is present, then its charge needs to be taken into consideration). In more general terms, the parallel circuit including the buffer capacitor C33 and the associated LED cascade is supplied with power from that point onward.
In the further course of the half-cycle, the voltage between the nodes 703 and 704 continues to rise, as a result of which the potential on the node N33 increases further accordingly. By means of the LEDs D300 to D313 that are on, the potential on the node N32 therefore also rises. The voltage difference between the potential on the node N33 and on the node N32 is 47.3V−4.6V=42.7V, or in simpler terms (14*3+0.7)V=42.7V. The capacitor C22 is charged to 28*3V=84V (the forward voltage of the 28 diodes D200 to D227).
If the rectified AC voltage rises to 60V, then these 60V are present on the node N23, since all switches Q11, B11 situated above the latter are on. The voltage on the node N25 is therefore 60V−84V=−24V. Since the voltage on the node N6 is constantly 6V, the switches Q21 and B21 are on, and the potentials of the nodes N22, N24, N33 are therefore coupled to that of N23 at low impedance. When the input voltage rises further, the potential on the node N23 and hence the potential on the node N24 increase. When the potential on the node N25 has reached 4.6V (potential on the node N6 minus the forward voltage of the diode D21 minus UBE of Q21), the switch Q21 and hence the switch B21 change to the off state. This is the case when the potential on the node N23 has reached 88.6V (4.6V on the node N33 plus 28 times 3V). From this time onward, the current begins to flow via the LED cascade D200 to D227 of the LED unit LE2, or the parallel circuit including LED cascade D200 to D227 and buffer capacitor C23 is supplied with power. At an input voltage of 88.6V, 28 times 3V plus 0.7V (the forward voltages of the 28 LEDs D200 to D227 and the forward voltage of the diode D23) are therefore dropped, as seen from the outside, across the LED unit LE2, so that the potential on the node N22 is now only 3.9V. Since the node N22 corresponds to the node N33, the potential on the node N33 therefore also is now just 3.9V. The potential on the node N35 is accordingly 3.9V−42.0V (in line with the potential on the node N33 minus the voltage stored in the capacitor C32)=−38.1V. Therefore, the voltage difference between the node N6 and the node N35 is −44.1V, as a result of which the transistor Q31 and hence the switch B31 turn on again. In this manner, the LED cascade D300 to D313 of the LED unit LE3 is shorted again from the outside, i.e. it is no longer supplied with power.
Accordingly, the LED cascades of the LED units LE2 and LE1 are supplied with power—the applicable switching phases are denoted by “1” in the table below—or shorted from the outside, which is denoted by “0” below.
SwitchingprocessLE1LE2LE310002001301040115100610171108111911110110111011210013011140101500116000
The switching processes 1 and 9 from the table above are not genuine switching processes, as can be seen from the respective state equality of the switching processes 16 and 8. Rather, they depict important times for the rectified mains voltage applied to the input of the whole circuit: the transition from 16 to 1 denotes the minimum of the input voltage at the time of the mains zero crossing, at which no switching states change, but, on account of the abruptly changing rise, the logic does indeed. The same applies to the transition from 8 to 9, which describes the maximum of the input voltage. Thereafter, the reverse effect begins, i.e. the LED cascades of the LED units LE1, LE2 and LE3 are switched in succession, in precisely the reverse order, in line with the table above until, at a phase angle of 180°, all LED cascades are bypassed again (B11 to B31 on) and a new half-cycle begins. When the instantaneous supply voltage falls, the case frequently arises, for example, that a higher LED unit is bypassed and, as a result, the one directly beneath is subsequently supplied with power again.
To produce an auxiliary voltage for the node N6, the voltage drop across the voltage equalization series impedance 12, i.e. the voltage on the node N32, is used in the present case. On account of the binary design of the LED cascades, a sawtooth-like voltage becomes able to be tapped off therefrom, said voltage fluctuating between 3.9V (at every mains zero crossing 0V) and 46.6V until all LED cascades are connected. When all LED cascades are activated, that is to say during the above switching phases 8 and 9, a voltage is dropped across the voltage equalization series impedance 12, which voltage is obtained from the difference between the input voltage and the sum of the voltages dropped across the LED cascades and the sum of the forward voltages of the decoupling diodes. Since the voltage peaks in this sawtooth-like voltage are well distributed over time within a half-cycle, this sawtooth-like voltage can be used to produce an auxiliary voltage by means of an RC element R4, C2 and also a charging diode D3 and a zener diode D2 as voltage limiter. This auxiliary voltage has only low residual ripple, for which reason it is possible to use very small capacitances in comparison with other auxiliary power supplies. It is of very simple design and able to be produced in compact form, that is to say extremely inexpensive. Of particular advantage is the circumstance that, for the auxiliary power supply, a current is drawn that would otherwise be converted into power loss in the voltage equalization series impedance 12. Consequently, a parasitic power is used to produce the auxiliary voltage on the node N6. In this way, the auxiliary power supply produces no additional power loss, and the efficiency of the circuit arrangement is improved.
A disadvantage of the circuit arrangement depicted in FIG. 1a is the circumstance that it undesirably results in brighter and darker areas in a luminaire. In order to avoid impairing the appearance of an LED arrangement, DE 20 2013 000 064 U1 proposes, in this context, an LED arrangement having at least one first and a second LED chain, wherein the LEDs are arranged at least in part on at least one first arrangement area, and wherein the LEDs of the at least one first and second LED chain are arranged according to a prescribed criterion on the basis of their mean currents. A disadvantage of such an arrangement, however, is distinctly more complex conductor track routing, which can present difficulties particularly as regards compliance with the regulations concerning electromagnetic compatibility (EMC), and also low flexibility for adaptation to suit changed circuit parameters.